1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing the same, and more particularly, to a solar cell having a laminate structure of group-III nitride semiconductors (hereinafter, referred to as group-III nitride compound semiconductors) capable of achieving both mass productivity and excellent characteristics and to a method of manufacturing the same.
2. Description of the Related Art
It has been about thirty years since a technique for a solar photovoltaic system was developed, and energy consumption has rapidly increased all over the world. Meanwhile, most the countries ratified the Kyoto protocol in order to reduce greenhouse gases that cause climate change, and thus many countries have turned their attention to natural energy, particularly, solar photovoltaic power generation.
In particular, the solar photovoltaic market has been rapidly expanding since each country subsidized the development of the solar photovoltaic system. Due to an increase in the demand for silicon caused by the development of the semiconductor industry, a shortage in the supply of a silicon raw material has occurred in the solar photovoltaic market.
That is, with a rapid increase in demand for a polycrystalline silicon solar cell, which is the main product, a failure has occurred in the production plan of a crystalline raw material, and it is expected that a shortage in supply of the silicon raw material will occur in the near future. For this reason, the demand for reducing the thickness of silicon substrates has increased.
As a thin film solar cell, a solar photovoltaic system using, for example, an amorphous silicon thin film or a microcrystalline silicon thin film, has been known. In terms of development, a solar photovoltaic system using a compound semiconductor thin film formed of, for example, a CIS-based thin film having Cu, In, or Se as a base material, or a CIGS-based thin film containing Ga, has been paid attention.
Further, a solar photovoltaic system using a dye sensitization-type element containing a Ru-based pigment or an organic thin film element, which enables a flexible solar photovoltaic element, has been developed.
In order to expand the solar photovoltaic market, it is necessary to improve photoelectric conversion efficiency and reduce manufacturing costs. An InGaP/GaAs multi junction solar cell using a GaAs tunnel junction layer, which has been used as a power source for, for example, a satellite, has photoelectric conversion efficiency considerably higher than a silicon-based solar cell. However, has manufacturing costs are astronomically higher than the silicon-based solar cell. Therefore, it is difficult to use the In GaP/GaAs-based multi junction solar cell for general household purposes.
As examples of the solar cell using the compound semiconductor, the following have been proposed: a method of depositing a semiconductor multi-layer structure required for a multi-junction solar cell using an MOCVD method, thereby manufacturing a solar cell (for example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2001-4445)); a 3-terminal multi-junction solar cell formed by depositing Ge layers as a lower cell and GaAs layers as an upper cell using the MOCVD method (for example, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2002-368238)); and a concentrating photovoltaic apparatus having a concentrator in an InGaP/InGaAs/Ge based multi junction solar cell (for example, Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2006-313810)).
As a multi-junction tandem solar cell using a GaN-based material, the following has been proposed: a multi-junction tandem solar cell including a plurality of p-n junction layers that are formed by depositing In(1-x)Ga(x)N (Eg is in a range of about 0.7 eV to 3.4 eV)-based thin films using an MBE method. However, details of a manufacturing method and performances thereof are not disclosed (for example, Patent Document 4 (U.S. Published Application No, 2004-0118451)).
The solar photovoltaic systems using the compound semiconductors according to the related art have problems in that methods of depositing solar cell element thin films are complicated, there is no disclosure of a method of increasing the size of an element substrate, power consumption is considerably higher than other energy sources, and the manufacturing costs are higher than those of the polycrystalline silicon solar cell according to the related art.
Meanwhile, as a method of growing crystals of a group-III nitride compound semiconductor, the following have been proposed: an MOCVD method of reacting ammonia with an organic metal, such as organic gallium or an organic indium compound, at a high temperature during the manufacture of a light emitting element; an MBE method; and a sputtering method (for example, Patent Document 5 (Japanese Unexamined Patent Application Publication No. 60-39819), Non-Patent Document 1 (Ushiku Yukiko et al., Deposition of GaN Thin Film Using RF Magnetron Sputtering Method (I), Second 21st Century Joint Symposium-Science and Technology, and Human Being-(2003, Tokyo)) and Non-Patent Document 2 (Asami Akinori et al., Growth of GaN Single Crystal doped with Si or Mg by UHV Sputtering Method, 66th Meeting of the Japanese Society of Applied Physics (The University of Tokushima, autumn 2005)). However, the method of growing crystal using the MBE method or the sputtering method has not been industrially used for a light emitting element of the compound semiconductor or the manufacture of a solar cell element.